I was reading this article about kegging and it mentions the following:
You can now disconnect your airline, leaving the 30 psi in the keg, and leave your keg at room temperature for a week or two (or more). This is usually a good idea if your beer was racked in directly from the primary fermenter, or if you are dry-hopping, or if you are lagering the beer (in the keg) in a cold environment. The beer will be safe and sound for months and months left in there. (Assuming you've left 30 psi in the headspace, you have no leaks and your temperatures are not extraordinarily high.) Your beer will NOT carbonate under these conditions, and eventually you will have to do one of the steps below.
Why, if I leave 30 psi in the keg (without the line attached) why will the beer not carbonate under these conditions?